Working of metal bodies

ABSTRACT

Electrolytic working of metal or mineral bodies. An electrolytic circuit is established using nitric acid as electrolyte and cathodes which are inert to the electrolyte under the operating conditions and the body to be worked is immersed into the electrolyte and brought into direct contact with, or close proximity to, the cathode. A current is flown through the circuit and by this the body is bored, cut or the like according to the specific arrangement.

United States Patent Raviv et al.

[451 Jan. 18, 1972 [54] WORKING OF METAL BODIES [72] Inventors: SamuelRaviv, Beer-Sheva; Elsa Rabinovitz, Dimona; Shimon Malkiely, Beer-Sheva,all of Israel [73] Assignee: The State of Israel, Atomic EnergyCommission, Beer-Sheva, Israel [22] Filed: Aug.tl2, 1968 [21] Appl.No.:751,925

[30] I Foreign Application Priority Data Feb. 29, 1968 Israel ..29,547

[5 6] References Cited UNITED STATES PATENTS 2,990,282 6/1961 Wicke..204/143 X 3,139,394 6/1964 0elgoetz.... ..204/143 3,485,731 12/1969Yokozawa et al ..204/143 X Primary Examiner-Daniel E. Wyman AssistantExaminer-C. F. Dees Attorney-Browdy and Niemark [57] ABSTRACT 9 Claims,1 Drawing Figure WORKING or METAL BODIES The present invention concernsa method for working metals, metal alloys and minerals.

In the context of the present specification, the term working is used ina general way and is meant to imply various kinds of shaping operationsthat are conventionally brought about by mechanical means such asboring, cutting, reaming, engraving and the like.

Working of metals, especially hard metals, such as various nonofidablesteels, various nickel alloys, uranium and the like, as well as of hardminerals, requires cumbersome equipment with special hard-tipped toolswhich, in view of the high abrasion and wear and tear, have to bereplaced frequently. Moreover, the energetic requirements for workinghard metals and minerals are high.

It is the object of the present invention to provide simpler and cheapermeans for working metals and minerals.

Recent work by the same inventors on certain electrolytic phenomena hasled to some surprising discoveries. Thus, in Belgian Pat. No. 704.471 ofthe 29th Sept., 1967, there is described a method of separating the coreof a uranium fuel element from its cladding which comprises insertingthe element into a nitric acid bath whose concentration is within therange of from 7 to 12 N, connecting the uranium core, while the elementis so immersed, as cathode into an electrolytic circuit, allowingelectric current to flow in the electric circuit until the core issufficiently loosened from the cladding and then extracting the core.The possibility to extract the uranium core from its metal cladding insuch a manner was completely unexpected since the measures employed,i.e., immersion into nitric acid and connection of the uranium core ascathode into an electrolytic circuit, are known each by themselves tocause passivation, a phenomenon which is antagonistic to dissolution.Yet by proceeding in the above manner dissolution of uranium occurs, inparticular at the uranium-cladding interphase.

In Belgian Pat. No. 704.472, of the 29th Sept., 1967, there is describeda method for the dissolution of electronegative metals and metal alloysthat normally are either not attacked at all or attacked only veryslowly by nitric acid. This method comprises the immersion of the metalinto nitric acid, connecting it while so immersed as cathode intoan'electrolytic circuit and allowing an electric current to flow in thecircuit until the desired degree of dissolution has occurred. Hereagain, the possibility to dissolve in this manner an electronegativemetal of the kind specified was surprising, again for the reason thateach expedient by itself, that is treatment with nitric acid andcathodic treatment, leads to passivation, that is to a phenomenonantagonistic to dissolution.

In Belgian Pat. No. 703.419, of the 4th Sept., 1967, there is describeda process for the production of nitrous oxide comprising establishing anelectrolytic circuit in which the elec' trolyte is nitric acid, oneelectrode is of an electronegative metal or metal alloy, which is notattacked at all or attacked only very slowly by nitric acid, and theother is of a conductive material inert to nitric acid under operationalconditions, and flowing an electric current through this circuit underconditions which do not cause dissolution of the electronegativeelectrode. Under these conditions a nitrous oxide containing gas mixtureevolves near the cathode and can be withdrawn therefrom.

All the above unexpected phenomena led to the assumption that they weredue to certain electric surface phenomena occurring at the interphasebetween metal bodies and nitric acid and that these phenomena can beinfluenced in a desired direction by an imposed electric current. Thisrecognition has then led to further investigations and surprisingdiscoveries which underlie the present invention.

The present invention consists in a method for working a metal ormineral body, comprising establishing an electrolytic circuit in whichthe electrolyte is nitric acid and the electrodes are inert to theelectrolyte-under the operational conditions, provided that the cathodeis of a different material than said body, introducing said body intothe electrolyte in a proximity relationship with the cathode, flowing aunidirectional electric current through the circuit thereby to bringabout local dis solution of the body, maintaining theproximityrelationship of the body with the cathode until the desireddegree of local dissolution is achieved, and then withdrawing the bodyfrom the electrolyte.

If desired, the system may comprise two or more cathodes connected inparallel.

The term proximity relationship" is used herein to describe the distancebetween the worked body and the cathode and is meant to imply eitherdirect physical contact or a spaced relationship not exceeding 10millimeters. The description local dissolution" implies dissolutionwithin the zone, that is, in proximity relationship with a cathode.

Within the scope of the above general method many different embodimentsare conceivable. Thus, in accordance with one embodiment, the body isbored by placing one or more oblong cathodes each in proximityrelationship to said body. In accordance with this embodiment it may benecessary to move gradually the body and the cathode towards each otheras the boring proceeds.

In accordance with another embodiment of the invention, the diameter ofan existing bore is widened by placing an oblong cathode so as to extendtherethrough in axial direction and the electric current is made to flowuntil the bore is widened to the desired diameter.

In accordance with yet another embodiment of the invention, a metal ormineral body is cut or engraved by bringing one or more cathodes into aproximity relationship with the body along a predetermined cutting lineor lines. In carrying out this embodiment it is possible to move apointed cathode along the cutting line or to wind a flexible cathode onthe body along the line. Also in this embodiment a gradual movement ofthe body and each cathode towards each other as the operation proceedsmay be necessary.

Many other embodiments are conceivable within the scopeof the presentinvention, all being variations within the frame of the above generalprinciple.

The electrodes used in carrying out the method according to theinvention may be of any suitable material provided they are notchemically attacked by the nitric acid under the operating conditions.

There are no critical limitations for the temperature of the nitric acidbath that is used in carrying out the method according to the inventionand any temperature between the freezing and boiling points may beemployed. Wherever possible it will be preferable to operate at room oronly slightly elevated temperature.

Likewise, there are also no critical limitations for the concentrationof the nitric acid and/or the current intensity. Both parameters have tobe selected in accordance with the properties of the body to be worked,taking, however, into consideration that both the body and theelectrodes must not be attacked chemically.

The method according to the invention differs basically from allhitherto known electrolytic processes. In known electrolytic processesthe system comprises only a cathode, an anode and an electrolytic bath.Where in such a process a body has to be worked in any way,electroplated, passivated or dissolved, e.g., in accordance with theabove-mentioned Belgian Pat. No. 704.472, the body itself forms one ofthe electrodes, e.g., the cathode. In consequence, where a body isworked in accordance with any known method and thus serves as anelectrode in the system, this electrode, i.e., the body, changes in thecourse of the treatment. In distinction therefrom, in the methodaccording to the invention, three elements are present in theelectrolytic bath, to wit, an anode, a cathode and the body that isbeing worked. The body, which is thus neither cathode nor anode, willfor the reason of its proximity relationship with the cathode, be termedhereinafter as anticathode." In a system according to the invention onlythe anticathode changes in accordance with the treatment whereascathodes and anodes remain substantially unaffected.

It is a characteristic feature of the present invention that thetreatment is completely topical and only a zone of the anticathode tatis in proximity relationship with the cathode is dissolved whereas theremaining part of the anticathode remains unaffected. It is due to thisfeature that a complete control of the process is possible and theanticathode may in accordance with the invention be shaped in anydesired way.

It is believed that the method according to the invention is based onlocal modifications of the electric surface properties of theanticathode, which modifications occur in the zone of close proximity tothe cathode. As a consequence of these modifications a zone of theanticathode that is in proximity relationship with a cathode isdepassivated to such an extent that it can be attacked chemically bynitric acid.

The invention is illustrated, by way of example only, in theaccompanying drawing which is a wiring diagram of an electrolytic systemfor carrying out the method according to the invention.

As shown in the drawing, an electrolytic system in accordance with theinvention comprises a vessel 1 holding a nitric acid bath 2. Into vessel1 dips a cathode 3 and an anode 4 and immersed in the bath andpositioned in proximity relationship with the tip of cathode 3 is ananticathode 5 which is the body to be worked in accordance with theinvention. As can be seen, contrary to common anodic dissolutionsystems, the anode 4 is not electrically connected to the work body 5.Cathode 3 is connected to the negative terminal and anode 4 to thepositive terminal of a battery 6. An ammeter A serves for measuring thecurrent intensity and a voltmeter V for voltage determination. Thesystem further comprises switching means as well as voltage and currentintensity controlling means which are all known per se and notillustrated in the wiring diagram.

Where it is desired, for example, to bore a hole in anticathode 5 the DCis switched on and the anticathode 5 and cathode 3 are gradually movedtowards each other as the boring proceeds. The operation continues aslong as desired and is interrupted by switching off the current. Theso-processed anticathode is then removed from the bath.

The invention is further illustrated by the following examples to whichit is not limited. In all the examples the electric current was DC.

EXAMPLE 1 A cylindrical uranium rod of 35-millimeter diameter and 80-mm.long was cut across. For this purpose a platinum wire was wound aroundthe circumferential line along which the cut was intended. The body wasplaced horizontally into a 1.5- liter bath of 4N HNO at 25 C. and theanode, equally a platinum wire, was so placed that its lower tip was 2cms. above the uranium rodv An electric current of 12 volts and ampereswas applied for 30 hours after which the rod was fully cut. During thisoperation the cathode, which formed a loop around the uranium rod, waspermanently tensioned.

EXAMPLE 2 A uranium rod of the same dimension as that used in example lwas bored across. To this end the loop-shaped cathode used in example 1was replaced by a straight, pointed cathode whose lower tip was directedin direction of the intended bore. The bath was again 1.5 liter of 4NHNO solution, the voltage applied was from 10 to 12 volts, the currentintensity from 5 to 8 amperes, and the temperature varied between and C.After 8.55 hours a diametrical bore was achieved whose inner diameterwas 4 mm.

The same procedure was repeated for boring an axial bore into the rod(80 mm. length) and this was achieved after 26.30 hours. The boreobtained in this way had an inner diameter of 2 mm.

EXAMPLE 3 A l-mm. thick wall of nonoxidizable steel pipe was bored. A

l-liter bath of a 7N HNO solution was employed, the voltage was 15volts, the current intensity 20 amperes and the tem- EXAMPLE 4 Anonoxidizable steel pipe measuring mm. in length and whose wall was 0.5mm. thick was cut in axial direction. For this purpose two cathodesconnected in parallel to the same DC source were used and applied at twodiametrically opposed points on the pipe. The cathode and anode wereboth of platinum. The electrolytic bath was 8N HNO, solution, thevoltage was 5 volts, the current intensity l0 amperes and thetemperature around 30 C. The two cathodes were gradually moved in axialdirection along the pipe and after a period of 45 minutes an axial cutof 35 mm. was produced.

EXAMPLE 5 The experiments of examples 3 and 4 were repeated withInconnel pipes (Inconnel is a trade name for an alloy containingapproximately 78 percent of nickel, 15 percent of chromium and 7 percentof iron) having the same respective dimensions. Similar results wereobtained.

EXAMPLE 6 A uranium rod measuring 35 mm. in diameter and 80 mm. inlength was bored using an aluminum cathode and platinum anode, a 4N HNOsolution, a current intensity of 4 amperes at a voltage of 10 volts. Theoperational temperature was 60 C. and after 6 hours a l2-mm. long borewas produced.

We claim:

1. A method for working a metal body which is chemically inert to nitricacid, comprising establishing an electrolytic circuit in which theelectrolyte is nitric acid and the electrodes are inert to theelectrolyte under the operating conditions provided that the cathode isof a different material than said body, introducing said body into theelectrolyte in a proximity relationship with the cathode and notelectrically connected to the anode, flowing a unidirectional electriccurrent through the circuit thereby to bring about local dissolution ofthe body, maintaining the proximity relationship of the body with thecathode until the desired degree of local dissolution is achieved, andthen withdrawing the body from the electrolyte.

2. A method according to claim 1, wherein at least two cathodes areapplied to said body.

3. A method according to claim 1, wherein said body is bored by placingat least one oblong cathode in proximity relationship thereto.

4. A method according to claim 1, wherein an existing bore in said bodyis widened by placing an oblong cathode so as to extend therethrough inaxial direction and the electric current is made to flow until the boreis widened to the desired diameter.

5. A method according to claim 1, wherein said body is cut or engravedby bringing at least one cathode into proximity relationship with saidbody along a predetermined line.

6. A method according to claim 5, wherein a pointed cathode is used andis moved along said line.

7. A method according to claim 5, wherein a flexible cathode is woundaround said line or part thereof.

8. A method according to claim 1, wherein said body and each cathode aregradually moved towards each other.

9. A method according to claim 7, wherein a cathode is wound on saidbody and is gradually tightened as the cutting proceeds.

2. A method according to claim 1, wherein at least two cathodes areapplied to said body.
 3. A method according to claim 1, wherein saidbody is bored by placing at least one oblong cathode in proximityrelationship thereto.
 4. A method according to claim 1, wherein anexisting bore in said body is widened by placing an oblong cathode so asto extend therethrough in axial direction and the electric current ismade to flow until the bore is widened to the desired diameter.
 5. Amethod according to claim 1, wherein said body is cut or engraved bybringing at least one cathode into proximity relationship with said bodyalong a predetermined line.
 6. A method according to claim 5, wherein apointed cathode is used and is moved along said line.
 7. A methodaccording to claim 5, wherein a flexible cathode is wound around saidline or part thereof.
 8. A method according to claim 1, wherein saidbody and each cathode are gradually moved towards each other.
 9. Amethod according to claim 7, wherein a cathode is wound on said body andis gradually tightened as the cutting proceeds.